Generally, a DC-DC converter utilizing a switching operation, e.g., by means of a Pulse-Width Modulation (PWM) control of a switching element such as a transistor has a relatively small size, and therefore can be embodied in an integrated circuit (IC). In addition, an inductor-capacitor (or L-C) filter is usually utilized as circuit components in order to achieve the averaging of the circuit in the DC-DC converter.
FIG. 1a shows a conventional example of a DC-DC converter with a PWM control circuit. The DC-DC converter 10 converts a DC input voltage Vin into another DC output voltage Vout. DC-DC converter 10 has a PWM control circuit 12 to turn ON and OFF the switch 14, e.g., a MOSFET. The averaging circuit 16, e.g., an L-C filter, then smoothes the node voltage Vsn, which is in a PWM waveform, to result in the steady DC output voltage Vout. The converter 10 also has an input capacitor 17. Since the current delivered to the converter 10 is an average DC current and the current in the switch 14 is pulsating, the input capacitor 17 is placed at the input of the converter 10 to average the input current. The input capacitor 17 provides a low-impedance voltage source for the converter 10 and helps to filter the pulsating current and to avoid the electromagnetic interference (EMI). For a detailed description of the converter 10, reference is made to Lynch and Hesse, Under the Hood of Low-Voltage DC/DC Converters, Texas Instruments Power Supply Design Seminar, SEM-1500, (SLUP184) (hereafter, referred as Lynch and Hesse).
When the switch 14 is turned ON and OFF in response to the PWN control signal of the circuit 12, the node voltage Vsn will rise or fall accordingly. But in practice, the DC input voltage Vin will be inevitably affected by the operation of the switch 14 and has some oscillations at switching moments. Particularly, as shown FIG. 1b, turning OFF of the switch 14 results in more noticeable oscillations (or so-called “rings”), which may have negative impacts back on the source of the DC input voltage which may supply to other devices at the same time. Typically, the system, like an electronic device, allows DC input voltage vary in a range. However, it will be a problem if the downside of the oscillating input voltage is lower than the minimum of the range.
Note that Lynch and Hesse make a reference that the rings of the node voltage rising could be controlled by a snubber circuit. For a description of the snubber circuit, reference is made to Philip Todd, Snubber Circuits: Theory, Design, and Application, SEM-900 Unitrode Power Supply Design Seminar, 1994. Topic II, TI Literature No. SLUP100. However, Lynch and Hesse have not discussed the rings of the input voltage. Nor have they suggested adopting a snubber circuit to reduce the rings of the input voltage.